Silicon wafers are currently manufactured in a sequence of steps, where each step places a pattern of material on the wafer. By building up successive layers of patterned metal, patterned insulators, patterned photoresist, etc.; transistor structures and/or other structures can be formed. In order for a final device to function correctly, these patterns for successive layers must be aligned correctly. For example, a contact layer, which resides vertically between an upper metal layer and a lower contact area, must be laterally arranged within a lateral width of the upper metal line and lower contact area, such that the contact layer when formed extends vertically between the metal line and contact area to form an electrical connection there between (e.g., ohmic connection). Misalignment of any kind can cause short circuits and/or connection failures, which in turn impact fab yield and profit margins.
Overlay control, sometimes abbreviated as OVL, defines the control of this pattern-to-pattern alignment between different layers. It plays an important role in semiconductor manufacturing, helping to monitor layer-to-layer alignment on multi-layer device structures.